Drug resistance of tumor cells occurring during cancer chemotherapy is an important problem facing clinicians and research scientists interested in the treatment of malignant neoplasms. The mechanisms underlying the development of drug resistance are known to be diverse. Our understanding of events on the molecular level related to drug resistance, however, remains unsatisfactory. Our efforts will focus on this gap by combining techniques of cell biology, biochemistry and molecular biology. The ultimate objective of this proposal is to obtain clinically relevant data on the role, if any, that DNA hypermethylation occurring within drug-arrested replication forks may play in the development of drug-resistant phenotypes. This investigation will focus upon two major questions: (1) Is DNA hypermethylation occurring within inhibited replication for ks the biological cause of gene amplification? (2) Is drug-induced DNA hypermethylation responsible for inactivation of genes whose products are required for drug sensitivity? Answers to these questions may permit an approach to cancer chemotherapy that allows better advanced planning to avoid drug resistance and quicker response times once resistant tumor cells have been identified. Preliminary studies strongly suggest that DNA hypermethylation occurring within drug-arrested replication forks may play a major role in the development of drug-resistant phenotypes.